Mask with multilayer structure and manufacturing method by using the same

ABSTRACT

The present disclosure provides a photolithography mask. The photolithography mask includes a substrate that contains a low thermal expansion material (LTEM). A multilayer (ML) structure is disposed over the substrate. The ML structure is configured to reflect radiation. The ML structure contains a plurality of interleaving film pairs. Each film pair includes a first film and a second film. The first film and the second film have different material compositions. Each film pair has a respective thickness. For at least a subset of the plurality of the film pairs, the respective thicknesses of the film pairs change randomly along a predefined direction.

PRIORITY DATA

This application is a continuation of U.S. patent application Ser. No.14/801,914, filed Jul. 17, 2015, herein incorporated by reference in itsentirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing. For these advances to be realized,similar developments in IC processing and manufacturing are needed. Forexample, the need to perform higher resolution lithography processesgrows. One lithography technique is extreme ultraviolet lithography(EUVL). Other techniques include X-Ray lithography, ion beam projectionlithography, electron beam projection lithography, and multiple electronbeam maskless lithography.

The EUVL employs scanners using light in the extreme ultraviolet (EUV)region, having a wavelength of about 1-100 nm. Some EUV scanners provide4× reduction projection printing, similar to some optical scanners,except that the EUV scanners use reflective rather than refractiveoptics. i.e., mirrors instead of lenses. EUV scanners provide thedesired pattern on an absorption layer (“EUV” mask absorber) formed on areflective mask. Currently, binary intensity masks (BIM) are employed inEUVL for fabricating integrated circuits. EUVL is similar to opticallithography in that it needs a mask to print wafers, except that itemploys light in the EUV region. i.e., at 13.5 nm. At the wavelength of13.5 nm or so, all materials are highly absorbing. Thus, reflectiveoptics rather than refractive optics is used. A multilayered (ML)structure is used as a EUV mask blank.

However, conventional EVU masks and the fabrication thereof may stillhave drawbacks. For example, the ML structure in conventional EUV masksis a periodic structure, which may include 40 pairs of silicon andmolybdenum. Each pair of silicon and molybdenum has the same thicknessas the rest of the pairs. The periodic ML structure may lead to lowreflectivity and diffraction imbalance, which are undesirable.

Therefore, while conventional EUV lithography systems and processes havebeen generally adequate for their intended purposes, they have not beenentirely satisfactory in every aspect. What is needed is a EUVlithography method and system having improved reflectivity anddiffraction balance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic view of a lithography system constructed inaccordance with some embodiments.

FIG. 2 is a sectional view of a EUV mask constructed in accordance withsome embodiments.

FIG. 3 is a sectional view of a multilayer structure of the EUV maskconstructed in accordance with some embodiments.

FIG. 4 is a sectional view of a semiconductor wafer undergoingprocessing in accordance with some embodiments.

FIG. 5 is a flowchart of a method of fabricating a EUV mask inaccordance with some embodiments.

FIG. 6 is a flowchart of a method of wafer manufacturing process inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a schematic view diagram of a lithography system 10,constructed in accordance with some embodiments. The lithography system10 may also be generically referred to as a scanner that is operable toperform lithography exposure processes with respective radiation sourceand exposure mode. In the present embodiment, the lithography system 10is an extreme ultraviolet (EUV) lithography system designed to expose aresist layer by EUV light. The resist layer is a material sensitive tothe EUV light. The EUV lithography system 10 employs a radiation source12 to generate EUV light, such as EUV light having a wavelength rangingbetween about 1 nm and about 100 nm. In one particular example, theradiation source 12 generates a EUV light with a wavelength centered atabout 13.5 nm. Accordingly, the radiation source 12 is also referred toas EUV radiation source 12.

The lithography system 10 also employs an illuminator 14. In variousembodiments, the illuminator 14 includes various refractive opticcomponents, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors in order to direct light from the radiation source 12onto a mask stage 16, particularly to a mask 18 secured on the maskstage 16. In the present embodiment where the radiation source 12generates light in the EUV wavelength range, the illuminator 14 employsreflective optics. In some embodiments, the illuminator 14 includes adipole illumination component.

In some embodiments, the illuminator 14 is operable to configure themirrors to provide a proper illumination to the mask 18. In one example,the mirrors of the illuminator 14 are switchable to reflect EUV light todifferent illumination positions. In some embodiment, a stage prior tothe illuminator 14 may additionally include other switchable mirrorsthat are controllable to direct the EUV light to different illuminationpositions with the mirrors of the illuminator 14. In some embodiments,the illuminator 14 is configured to provide an on-axis illumination(ONI) to the mask 18. In an example, a disk illuminator 14 with partialcoherence σ being at most 0.3 is employed. In some other embodiments,the illuminator 14 is configured to provide an off-axis illumination(OAI) to the mask 18. In an example, the illuminator 14 is a dipoleilluminator. The dipole illuminator has a partial coherence σ of at most0.3 in some embodiments.

The lithography system 10 also includes a mask stage 16 configured tosecure a mask 18. In some embodiments, the mask stage 16 includes anelectrostatic chuck (e-chuck) to secure the mask 18. This is because gasmolecules absorb EUV light, and the lithography system for the EUVlithography patterning is maintained in a vacuum environment to avoidthe EUV intensity loss. In the disclosure, the terms of mask, photomask,and reticle are used interchangeably to refer to the same item.

In the present embodiment, the lithography system 10 is a EUVlithography system, and the mask 18 is a reflective mask. One exemplarystructure of the mask 18 is provided for illustration. The mask 18includes a substrate with a suitable material, such as a low thermalexpansion material (LTEM) or fused quartz. In various examples, the LTEMincludes TiO₂ doped SiO₂, or other suitable materials with low thermalexpansion.

The mask 18 also includes a reflective ML deposited on the substrate.The ML includes a plurality of film pairs, such as molybdenum-silicon(Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layerof silicon in each film pair). Alternatively, the ML may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light. Conventionally,the Mo/Si film pairs or Mo/Be film pairs in EUV masks are distributedperiodically. In other words, each film pair has substantially equalthickness as each of the other film pairs. According to the variousaspects of the present disclosure, however, the ML structure includesaperiodic film pairs to improve reflectivity and diffraction balance, asdiscussed in greater detail below.

The mask 18 may further include a capping layer, such as ruthenium (Ru),disposed on the ML for protection. The mask 18 further includes anabsorption layer deposited over the ML. The absorption layer ispatterned to define a layer of an integrated circuit (IC), the absorberlayer is discussed below in greater detail according to various aspectsof the present disclosure. Alternatively, another reflective layer maybe deposited over the ML and is patterned to define a layer of anintegrated circuit, thereby forming a EUV phase shift mask.

The lithography system 10 also includes a projection optics module (orprojection optics box (POB) 20 for imaging the pattern of the mask 18 onto a semiconductor substrate 26 secured on a substrate stage 28 of thelithography system 10. The POB 20 has refractive optics (such as for UVlithography system) or alternatively reflective optics (such as for EUVlithography system) in various embodiments. The light directed from themask 18, diffracted into various diffraction orders and carrying theimage of the pattern defined on the mask, is collected by the POB 20.The POB 20 may include a magnification of less than one (thereby thesize of the “image” on a target (such as target 26 discussed below) issmaller than the size of the corresponding “object” on the mask). Theilluminator 14 and the POB 20 are collectively referred to as an opticalmodule of the lithography system 10.

The lithography system 10 also includes a pupil phase modulator 22 tomodulate optical phase of the light directed from the mask 18 so thatthe light has a phase distribution on a projection pupil plane 24. Inthe optical module, there is a plane with field distributioncorresponding to Fourier Transform of the object (the mask 18 in thepresent case). This plane is referred to as projection pupil plane. Thepupil phase modulator 22 provides a mechanism to modulate the opticalphase of the light on the projection pupil plane 24. In someembodiments, the pupil phase modulator 22 includes a mechanism to tunethe reflective mirrors of the POB 20 for phase modulation. For example,the mirrors of the POB 20 are switchable and are controlled to reflectthe EUV light, thereby modulating the phase of the light through the POB20.

In some embodiments, the pupil phase modulator 22 utilizes a pupilfilter placed on the projection pupil plane. A pupil filter filters outspecific spatial frequency components of the EUV light from the mask 18.Particularly, the pupil filter is a phase pupil filter that functions tomodulate phase distribution of the light directed through the POB 20.However, utilizing a phase pupil filter is limited in some lithographysystem (such as a EUV lithography system) since all materials absorb EUVlight.

As discussed above, the lithography system 10 also includes thesubstrate stage 28 to secure a target 26 to be patterned, such as asemiconductor substrate. In the present embodiment, the semiconductorsubstrate is a semiconductor wafer, such as a silicon wafer or othertype of wafer. The target 26 is coated with the resist layer sensitiveto the radiation beam, such as EUV light in the present embodiment.Various components including those described above are integratedtogether and are operable to perform lithography exposing processes. Thelithography system 10 may further include other modules or be integratedwith (or be coupled with) other modules.

The mask 18 and the method making the same are further described inaccordance with some embodiments. In some embodiments, the maskfabrication process includes two operations: a blank mask fabricationprocess and a mask patterning process. During the blank mask fabricationprocess, a blank mask is formed by deposing suitable layers (e.g.,reflective multiple layers) on a suitable substrate. The blank mask isthen patterned during the mask patterning process to achieve a desireddesign of a layer of an integrated circuit (IC). The patterned mask isthen used to transfer circuit patterns (e.g., the design of a layer ofan IC) onto a semiconductor wafer. The patterns can be transferred overand over onto multiple wafers through various lithography processes. Aset of masks is used to construct a complete IC.

The mask 18 includes a suitable structure, such as a binary intensitymask (BIM) and phase-shifting mask (PSM) in various embodiments. Anexample BIM includes absorptive regions (also referred to as opaqueregions) and reflective regions, patterned to define an IC pattern to betransferred to the target. In the opaque regions, an absorber ispresent, and an incident light is almost fully absorbed by the absorber.In the reflective regions, the absorber is removed and the incidentlight is diffracted by a multilayer (ML). The PSM can be an attenuatedPSM (AttPSM) or an alternating PSM (AltPSM). An exemplary PSM includes afirst reflective layer (such as a reflective ML) and a second reflectivelayer patterned according to an IC pattern. In some examples, an AttPSMusually has a reflectivity of 2%-15% from its absorber, while an AltPSMusually has a reflectivity of larger than 50% from its absorber.

One example of the mask 18 is shown in FIG. 2. The mask 18 in theillustrated embodiment is a EUV mask, and includes a substrate 30 madeof a LTEM. The LTEM material may include TiO₂ doped SiO₂, and/or otherlow thermal expansion materials known in the art. In some embodiments, aconductive layer 32 is additionally disposed under on the backside ofthe LTEM substrate 30 for the electrostatic chucking purpose. In oneexample, the conductive layer 32 includes chromium nitride (CrN), thoughother suitable compositions are possible.

The EUV mask 18 includes a reflective multilayer (ML) structure 34disposed over the LTEM substrate 30. The ML structure 34 may be selectedsuch that it provides a high reflectivity to a selected radiationtype/wavelength. The ML structure 34 includes a plurality of film pairs,such as Mo/Si film pairs (e.g., a layer of molybdenum above or below alayer of silicon in each film pair). Alternatively, the ML structure 34may include Mo/Be film pairs, or any materials with refractive indexdifference being highly reflective at EUV wavelengths. As discussedabove, the ML structure in conventional EUV masks utilize periodic Mo/Sior Mo/Be film pairs, which may lead to insufficient reflectivity and/orexcessive diffraction imbalance. To remedy these shortcomings, the MLstructure 34 of the present disclosure utilizes aperiodic film pairs, asdiscussed below in greater detail with reference to FIG. 3.

Still referring to FIG. 2, the EUV mask 18 also includes a capping layer36 disposed over the ML structure 34 to prevent oxidation of the ML. Inone embodiment, the capping layer 36 includes silicon with a thicknessranging from about 4 nm to about 7 nm. The EUV mask 18 may furtherinclude a buffer layer 38 disposed above the capping layer 36 to serveas an etching-stop layer in a patterning or repairing process of anabsorption layer, which will be described later. The buffer layer 38 hasdifferent etching characteristics from the absorption layer disposedthereabove. The buffer layer 38 includes ruthenium (Ru), Ru compoundssuch as RuB, RuSi, chromium (Cr), chromium oxide, and chromium nitridein various examples.

The EUV mask 18 also includes an absorber layer 40 (also referred to asan absorption layer) formed over the buffer layer 38. In someembodiments, the absorber layer 40 absorbs the EUV radiation directedonto the mask. In various embodiments, the absorber layer may be made oftantalum boron nitride (TaBN), tantalum boron oxide (TaBO), or chromium(Cr), Radium (Ra), or a suitable oxide or nitride (or alloy) of one ormore of the following materials: Actium, Radium, Tellurium, Zinc,Copper, and Aluminum.

Referring now to FIG. 3, the aperiodic ML structure 34 of the presentdisclosure is described in more detail. FIG. 3 illustrates adiagrammatic fragmentary cross-sectional side view of a portion of theaperiodic ML structure 34. The aperiodic ML structure 34 includes aplurality of pairs of Mo/Si films or layers. Radiation such as light inthe EUV range is projected toward the topmost surface of the aperiodicML structure 34 in FIG. 3. In the embodiment illustrated in FIG. 3, thepairs of Mo/Si films are divided into groups or subsets, wherein theperiod (measured by thickness) gradually or randomly changes within eachgroup or subset.

For example, the ML structure 34 includes a group 100A that containsthree pairs of Mo/Si film. The first pair of Mo/Si film includes a Sifilm 110 disposed on a Mo film 120. The second pair of Mo/Si filmincludes a Si film 111 disposed on a Mo film 121. The third pair ofMo/Si film includes a Si film 112 disposed on a Mo film 122. As isillustrated, the third pair of Mo/Si film 112/122 is disposed over thesecond pair of Mo/Si film 111/121, which is disposed over the first pairof Mo/Si film 110/120.

The first pair of Mo/Si film has a total thickness 150. The second pairof Mo/Si film has a total thickness 151. The third pair of Mo/Si filmhas a total thickness 152. According to the various aspects of thepresent disclosure, the thickness 150, 151, and 152 of the first,second, and third Mo/Si film pairs change gradually or randomly along apredefined direction, for example along a vertical direction in FIG. 3,which in FIG. 3 may be defined as the direction in which the variousfilms are disposed on top of one another.

In some embodiments, the thickness 151 is equal to d, the thickness 150is in a range from about 0.5 d to d, and the thickness 152 is in a rangefrom about d to 1.5 d. In some other embodiments, the thickness 151 isequal to d, the thickness 150 is in a range from about 0.8 d to d, andthe thickness 152 is in a range from about d to 1.2 d. In yet otherembodiments, the thickness 151 is equal to d, the thickness 150 is equalto about 0.86 d, and the thickness 152 is equal to about 1.05 d. Inthese embodiments, d is a thickness value, for example 7 nanometers(nm).

These numeric ranges discussed above are configured to improve thereflectivity and diffraction balance. This is illustrated in Table 1below, which includes experimental data of reflectivity and diffractionimbalance performance for a mask with an aperiodic ML structure versusconventional masks with a periodic ML structure.

TABLE 1 Min. Reflectivity (%) at Reflection Ripple (%) (Max Period (nm)angle (°) within 13.47° R-Min R) 7  8.5 at 15.40 63.83 7.07  35 at 16.1838.31 7.14 51 at 0.00 21.38 Aperiodic 61 at 0.00 8.25

The rows 2-4 of Table 1 are data taken from conventional masks withperiodic ML structures, though the period (measured by thickness of eachMo/Si pair) is different for each mask. The last row of Table 1 is datataken from an embodiment of the mask of the present disclosure, where anaperiodic ML structure is implemented. The middle column of Table 1lists reflectivity data, measured by “Min reflectivity (%) at angle (°)within 13.47°.” The right column of Table 1 lists diffraction imbalancedata, measured by “Reflection Ripple (%) (Max R−Min R). As is clearlyshown in Table 1, the reflectivity (at 61%) of the mask with theaperiodic ML structure is greater than the conventional masks, while thediffraction imbalance (at 8.25%) is smaller (meaning that thediffraction balance is better) than the conventional masks.

According to the various aspects of the present disclosure, the nominalthickness (d) of the film pair in aperiodic ML structure complies withthe following criteria:

$\begin{matrix}{\delta = {\frac{4{\pi\left( {{n_{H}d_{H}\cos\;\theta_{H}} + {n_{L}d_{L}\cos\;\theta_{L}}} \right)}}{\lambda} = {2m\;\pi}}}\end{matrix}$where NA (numerical aperture)*sigma/M (reduction)=sin (theta)>0.03,d=d_(H) (thickness of higher layer)+d_(L) (thickness of lower layer),and theta (θ) is the incident angle of the radiation source with awavelength of lambda (λ). In some embodiments, the wavelength of lamdais 6.5 nm. In other embodiments, the wavelength of lamda is 13 nm.

In the embodiment shown in FIG. 3, the first, second, and third Mo/Sifilm pairs made up of films 110-112 and 120-122 may also be collectivelyreferred to as a stack and is herein designated with the referencenumeral 100A. Within the aperiodic ML structure 34, this stack of filmsis repeated, which as shown in FIG. 3 are designated with referencenumerals 100B . . . 100X. In other words, the stack (or subset/group)100B is made up of films 110-112 and 120-122 that are substantiallysimilar to the corresponding films in the stack 100A, as are the stacks(such as stack 100X) disposed thereabove. In this manner, thethicknesses of the Mo/Si film pairs within the stack 100B (or stack100X) vary gradually or randomly along the vertical direction(increasing as they go upwards in the embodiment shown in FIG. 3). Forexample, within the stack 100B, the Mo/Si film pair 112/122 has agreater thickness than the Mo/Si film pair 111/121, and the Mo/Si filmpair 111/121 has a greater thickness than the Mo/Si film pair 110/120.Another way of describing the gradually-changing thickness of the filmpairs is that, within each stack (e.g., 100A/100B/100X) of film pairs,the film pairs become thicker the farther away they are from the LTEMsubstrate.

Another characteristic of the configuration of the aperiodic MLstructure 34 shown in FIG. 3 is that, although the thickness of theMo/Si film pairs increases in the upwards vertical direction within aspecific stack 100A/100B/100C, the thickness would actually decrease atan interface between two adjacent stacks. For example, the topmost Mo/Sifilm pair (constituting films 112/122) of the stack 100A has a thickness152, which as discussed above is greater than d. The bottommost Mo/Sifilm pair (constituting films 110/120) of the stack 100B has a thickness150 and forms an interface with the topmost Mo/Si film pair of the stack100A. As discussed above, the thickness 150 is less than d and istherefore less than the thickness 152. This illustrates that, althoughthe thickness of the Mo/Si film pairs gradually or randomly increases inthe upwards vertical direction within any stack 100A/100B, etc., suchthickness would drop after “crossing over” to the next stack disposedthereabove. In other words, the thickness of the Mo/Si film pairs withinthe aperiodic ML structure 34 would gradually increase in the upwardsvertical direction, and then it would drop (e.g., upon entering thestack located above), and then gradually increase again, and then drop,etc. This pattern repeats until the outermost surface (the outer surfaceof the film 112 in the stack 100X in this case) of the aperiodic MLstructure 34 has been reached.

It is understood that the stack 100A/100B/100X discussed above need notbe limited to three film pairs. For example, in some embodiments, eachstack (or subset or group) of Mo/Si film pairs may include four, five,six, or any other number of film pairs, as long as the Mo/Si film pairthickness is gradually or randomly changing within the stack along thevertical direction. In an extreme example, the entire aperiodicstructure 34 is made up of Mo/Si film pairs having gradually or randomlychanging thicknesses along the vertical direction. In other words, theentire aperiodic structure 34 may be viewed as a single stack 100A asdiscussed above.

It is also understood that although the above discussions use Mo/Si asan example film pair for the aperiodic ML structure, the film pairmaterials are not limited to Mo/Si. For example, Mo/Be may be used as asuitable film pair in place of Mo/Si. It is also understood that thevarious aspects of the present disclosure are not limited to EUVlithography. For example, they may also apply to Beyond ExtremeUltraviolet (BEUV) lithography as well, where the radiation wavelengthis 6.× nm (e.g., 6.5 nm) instead of 13.5 nm used in EUV lithography.

FIG. 4 is a diagrammatic fragmentary cross-sectional side view of asemiconductor wafer undergoing a lithography process according to thevarious aspects of the present disclosure. A substrate 200 is provided.The substrate 200 may include a silicon material or another suitablesemiconductor material. A material layer 210 is formed over thesubstrate 200. A photoresist layer 230 is formed over the material layer210 to pattern the material layer. The photoresist layer 230 ispatterned into photoresist layer segments 230A and 230B through alithography process 250, which as discussed above may be a EUVlithography process or may be a BEUV lithography process. Thelithography process 250 is performed using a lithography mask that isimplemented as an embodiment of the mask 18 (FIG. 2), which includes anaperiodic ML structure as discussed above with reference to FIG. 3.Again, the aperiodic ML structure allows for improved reflectivity anddiffraction balance, which help improve lithography performance.

FIG. 5 is a simplified flowchart illustrating a method 400 offabricating a lithography mask according to an embodiment of the presentdisclosure. In some embodiments, the lithography mask is a EUV mask. Inother embodiments, the lithography mask is a BEUV mask.

The method 400 includes a step 410 of forming an aperiodic multilayer(ML) structure over a low thermal expansion material (LTEM) substrate.The aperiodic ML structure is configured to reflect radiation. Theforming of the aperiodic ML structure includes forming a plurality ofinterleaving film pairs that each include a first film and a second filmhaving a different material composition than the first film. The formingof the interleaving film pairs is performed such that: the aperiodic MLstructure includes a plurality of substantially identical stacks of filmpairs; and within each stack of film pairs, the film pairs becomethicker the farther away they are located from the LTEM substrate. Insome embodiments, each stack of the film pairs include a first filmpair, a second film pair located over the first film pair, and a thirdfilm pair located over the first film pair; the first film pair has afirst thickness in a range from about 0.5 d to about d; the second filmpair has a second thickness equal to d; and the third film pair has athird thickness in a range from about d to about 1.5 d. In someembodiments, d is about 7 nanometers. In some embodiments, the firstfilm of the film pair contains Mo, and the second film of the film paircontains Si or Be.

The method 400 also includes a step 420 of forming a capping layer overthe aperiodic ML structure. In some embodiments, the capping layercontains silicon.

The method 400 includes a step 430 of forming an absorber layer over thecapping layer.

It is understood that additional steps may be performed before, during,or after the steps 410-430 shown herein. For example, the method 400 mayinclude a step of forming a buffer layer between the capping layer andthe absorber layer. The buffer layer and the absorber layer havedifferent etching characteristics. Additional steps are not specificallydiscussed herein for reasons of simplicity.

FIG. 6 is a simplified flowchart illustrating a method 500 of performinga wafer manufacturing process according to an embodiment of the presentdisclosure.

The method 500 includes a step 510 of forming a material layer over asubstrate. In some embodiments, the material layer contains a dielectricmaterial. In some other embodiments, the material layer contains a metalmaterial.

The method 500 includes a step 520 of forming a photoresist layer overthe material layer. The photoresist layer may be formed by aspin-coating process.

The method 500 includes a step 530 of patterning the photoresist layerusing a lithography mask in a photolithography process. In someembodiments, the patterning step 530 comprises exposing the photoresistlayer to extreme ultraviolet (EUV) radiation or beyond extremeultraviolet (BEUV) radiation and thereafter developing the exposedphotoresist layer to form patterned photoresist features.

The lithography mask includes a substrate that contains a low thermalexpansion material (LTEM) and an aperiodic multilayer (ML) structuredisposed over the substrate. The aperiodic ML structure is configured toreflect radiation. The aperiodic ML structure contains a plurality ofinterleaving film pairs. Each film pair includes a first film and asecond film, the first film and the second film having differentmaterial compositions. For at least a group of the plurality of the filmpairs, each film pair has a different thickness than the other filmpairs in the group.

In some embodiments, the group of the film pairs include a first filmpair, a second film pair formed over the first film pair, and a thirdfilm pair formed over the first film pair; the first film pair has afirst thickness in a range from about 0.5 d to about d; the second filmpair has a second thickness equal to d; and the third film pair has athird thickness in a range from about d to about 1.5 d. In someembodiments, d is about 7 nanometers.

In some embodiments, the aperiodic ML structure includes multiplesubstantially identical groups of film pairs. Within each group, thethicknesses of the film pairs vary gradually vertically.

In some embodiments, for an entirety of the aperiodic ML structure, thethicknesses of the film pairs change gradually along a predefineddirection.

In some embodiments, the lithography mask further comprises: a cappinglayer disposed over the aperiodic ML structure; and an absorber layerdisposed over the capping layer. In some embodiments, the first film ofthe film pair contains Mo; and the second film of the film pair containsSi or Be.

It is understood that additional steps may be performed before, during,or after the steps 510-530 shown herein. For example, the method 500 mayinclude a step of directing extreme ultraviolet (EUV) or beyond extremeultraviolet (BEUV) radiation onto the lithography mask via anilluminator. The illuminator may contain a dipole illuminationcomponent. Additional steps are not specifically discussed herein forreasons of simplicity.

Based on the above discussions, it can be seen that the presentdisclosure offers various advantages in EUV/BEUV lithography. It isunderstood, however, that not all advantages are necessarily discussedherein, and other embodiments may offer different advantages, and thatno particular advantage is required for all embodiments. One advantageis that the aperiodic ML structure improves reflectivity and diffractionbalance. Whereas conventional lithography masks typically employ aperiodic ML structure where all the interleaving film pairs have equalthicknesses, the aperiodic ML structure disclosed herein employ filmpairs with gradually changing thicknesses. This allows the reflectivityto be relatively high, while keeping the diffraction imbalance to berelatively low, and it does not impose any restrictions on the numericalaperture. As a result, lithography performance is improved. Anotheradvantage is that the aperiodic ML structure can be formed and usedbased on existing process flow, as it does not require additional layersor different materials.

The present disclosure provides for a photolithography mask inaccordance with some embodiments. The photolithography mask includes asubstrate that contains a low thermal expansion material (LTEM). Anmultilayer (ML) structure is disposed over the substrate. The MLstructure is configured to reflect radiation. The ML structure containsa plurality of interleaving film pairs. Each film pair includes a firstfilm and a second film, the first film and the second film havingdifferent material compositions. Each film pair has a respectivethickness. For at least a subset of the plurality of the film pairs, therespective thicknesses of the film pairs change randomly along apredefined direction.

The present disclosure provides a wafer manufacturing process. Amaterial layer is formed over a substrate. A photoresist layer is formedover the material layer. The photoresist layer is patterned using alithography mask in a photolithography process. The lithography maskincludes a substrate that contains a low thermal expansion material(LTEM) and an multilayer (ML) structure disposed over the substrate. TheML structure is configured to reflect radiation. The ML structurecontains a plurality of interleaving film pairs. Each film pair includesa first film and a second film. The first film and the second filmhaving different material compositions. For at least a group of theplurality of the film pairs, each film pair has a different thicknessthan the other film pairs in the group.

The present disclosure provides for a method of fabricating aphotolithography mask. A multilayer (ML) structure is formed over a lowthermal expansion material (LTEM) substrate. The ML structure isconfigured to reflect radiation. The forming of the ML structureincludes forming a plurality of interleaving film pairs that eachinclude a first film and a second film having a different materialcomposition than the first film. The forming of the interleaving filmpairs is performed such that: the ML structure includes a plurality ofsubstantially identical stacks of film pairs; and within each stack offilm pairs, the film pairs become thicker the farther away they arelocated from the LTEM substrate. A capping layer is formed over the MLstructure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A structure, comprising: a first layer; a secondlayer disposed over the first layer; a third layer disposed over thesecond layer; and a fourth layer disposed over the third layer; wherein:the first layer, the second layer, the third layer, and the fourth layerare layers of a reflective structure of a lithography mask; the firstlayer and the second layer combined have a first thickness; the thirdlayer and the fourth layer combined have a second thickness that isdifferent from the first thickness; the first layer and the third layerhave a first material composition; and the second layer and the fourthlayer have a second material composition different from the firstmaterial composition.
 2. The structure of claim 1, wherein thelithography mask is an extreme ultraviolet (EUV) lithography mask. 3.The structure of claim 1, wherein the first material compositionsincludes molybdenum, and wherein the second material compositionincludes silicon or beryllium.
 4. The structure of claim 1, wherein thesecond thickness is greater than the first thickness.
 5. The structureof claim 1, further comprising: a fifth layer disposed over the fourthlayer, the fifth layer having the first material composition; and asixth layer disposed over the fifth layer, the sixth layer having thesecond material composition.
 6. The structure of claim 5, wherein thefifth layer and the sixth layer combined have a third thickness that isdifferent from the first thickness and the second thickness.
 7. Thestructure of claim 6, further comprising: a seventh layer disposed overthe sixth layer, the seventh layer having the first materialcomposition; and an eighth layer disposed over the seventh layer, theeighth layer having the second material composition.
 8. The structure ofclaim 7, wherein: the seventh layer and the eight layer combined have afourth thickness; the first thickness is less than the second thickness;the second thickness is less than the third thickness; and the fourththickness is less than the third thickness.
 9. The structure of claim 8,wherein the first thickness is equal to the fourth thickness.
 10. Amethod, comprising: obtaining a lithography mask that includes areflective structure for reflecting radiation; and performing alithography process using the lithography mask; wherein: the reflectivestructure includes a plurality of first film pairs and a plurality ofsecond film pairs; each of the first film pairs and the second filmpairs includes a first material and a second material different from thefirst material; and each of the second film pairs is thicker than eachof the first film pairs.
 11. The method of claim 10, wherein theperforming the lithography process comprises performing an extremeultraviolet (EUV) process.
 12. The method of claim 10, wherein: thefirst material includes molybdenum; and the second material includessilicon or beryllium.
 13. The method of claim 10, wherein the reflectivestructure further includes a plurality of third film pairs that eachinclude the first material and the second material, and wherein each ofthe third film pairs is thicker than each of the second film pairs. 14.The method of claim 10, wherein the reflective structure includes aplurality of identical stacks, and wherein each of the stacks includesone of the first film pairs and one of the second film pairs.
 15. Amethod, comprising: forming a first layer over a substrate, the firstlayer having a first material composition; forming a second layer overthe first layer, the second layer having a second material compositiondifferent from the first material composition, wherein the first layerand the second layer are formed to have a combined first thickness;forming a third layer over the second layer, the third layer having thefirst material composition; forming a fourth layer over the third layer,the fourth layer having the second material composition, wherein thethird layer and the fourth layer are formed to have a combined secondthickness different from the combined first thickness; and forming acapping layer or an absorber layer over the fourth layer.
 16. The methodof claim 15, further comprising, before the forming of the cappinglayer, repeating the forming of the first layer, the forming of thesecond layer, the forming of the third layer, and the forming of thefourth layer a plurality of times to form a multi-layer reflectivestructure.
 17. The method of claim 15, wherein the forming of the firstlayer includes forming the first layer over a substrate that containsTiO₂ doped SiO₂.
 18. The method of claim 15, wherein the first materialcompositions includes molybdenum, and wherein the second materialcomposition includes silicon or beryllium.
 19. The method of claim 15,further comprising: forming a fifth layer over the fourth layer, thefifth layer having the first material composition; and forming a sixthlayer over the fifth layer, the sixth layer having the second materialcomposition, wherein the fifth layer and the sixth layer are formed tohave a combined third thickness that is different from the combinedfirst thickness and the combined second thickness.
 20. The method ofclaim 19, wherein: the combined first thickness is smaller than thecombined second thickness; and the combined second thickness is smallerthan the combined third thickness.